Optimization of a Novel All-Vanadium Hybrid Redox Flow Battery/Mediator Supercapacitor

Abstract

The increasing demand for renewable energy is reliant upon progress in energy storage, which is essential in smart-grid and transportation applications. Redox flow batteries have been a promising path for these applications with their very high specific energy, but they suffer from low specific power, slow response, and a low charge rate. In contrast, mediator supercapacitors display high specific power and a fast response rate with a high charge rate, but relatively low specific energy. A hybrid flow-battery/supercapacitor is fabricated and operated under different conditions to provide high specific energy and power along with performance capabilities across a wide range of charge rates, utilizing both physical and chemical charging methods within the cell. The porous electrodes of the electrochemical cell consisted of the Vanadium mediators (VOSO4 and VCl3), supercapacitor carbon black powder, and a polymer binder. A commercial ion exchange membrane was used as the separator. This investigation focuses on the effects of flow rate, charge rate, and mediator concentration on the performance of the hybrid device. Potentiostatic EIS (EIS), Cyclic Charge Discharge (PWR), and Cyclic Voltammetry (CV) measurements were conducted under a wide range of flow rates and mediator concentrations. The electrochemical kinetics of the mediators were evaluated using linear polarization methods. The results indicate that at a given current for PWR tests and scan rate for CV tests, both the flow rate and mediator concentration greatly impact the charge/discharge behavior, the total energy, and the efficiency of the hybrid system. The system performs over a wide range of currents with repeated cycling. A transition between capacitor-like behavior and battery-like behavior is observed.